To brake a motor vehicle, the wheels of the motor vehicle are braked. In trucks in particular, respective brake cylinders are assigned to the wheel brakes of the wheels, wherein the necessary brake pressure is usually generated pneumatically. If the driver of the vehicle requests a braking via a brake pedal, in normal braking mode, the brake pressure is set depending on the driver brake request. In known brake systems, often a service brake valve is activated by means of the brake pedal, and manages the supply to the brake cylinders from a pressure medium store.
As an alternative to normal braking mode, the brake pressure is set by a control unit at the respective wheel brakes according to the specifications from the control unit. DE 10 2009 058 154 A1 discloses such a brake system, the control unit of which in pressure control mode sets the necessary brake pressures when corresponding braking needs are established. Such braking needs may for example be antilock braking interventions if a braking need is established at specific wheels. The known brake system also takes into account external brake requests, for example from a driver assistance system. Driver assistance systems, as systems executed separately from the control unit, send signals according to the external brake request to the control device of the brake system, for example via a databus. In this way, in pressure control mode, an external target deceleration value is predefined for the control unit, i.e. a value which represents the deceleration of the motor vehicle desired by the driver assistance system. If in pressure control mode, both external brake requests and also a driver brake request occur, i.e. the driver brakes in addition to the external brake request, the control unit adjusts the brake pressure at the respective wheel brakes according to a resulting target deceleration value of the vehicle deceleration. The control unit determines the resulting target deceleration value by linking the external target deceleration value according to the external brake request and a variable corresponding to the driver brake request.
In the known brake system, the driver brake request and the external brake request are executed additively. Alternatively, in the known brake system, in a “maximum” mode, the control unit forms only the maximum value of the target deceleration values already requested internally by the brake system and the external target deceleration values, wherein the external brake request is set only if it is higher than the internal brake request. By linking the external brake request and the driver brake request into a resulting brake pressure, it is ensured that an external brake request is set in pressure control mode even in the absence of a driver brake request.